Because you can easily control a device, like a relay/etc, that needs much higher current than a microcontroller I/O pin can supply. In general, a 5V pin cannot provide enough current to even drive a common 5V relay.

In addition, it allows you to control devices that require different voltage levels, such as a 12V relay.

That's for simple on-off control. If you want to actually amplify "signals", that's somethingelse again, in which case you want a more reliable amplification-value, see Emitter degeneration section here,http://en.wikipedia.org/wiki/Common_emitter